The invention relates to thermosetting adhesive and sealing substances of the composition of:
(a) 70 to 95 wt.-%, preferably 80 to 92 wt.-%, of polyesters having a glass transition temperature below 20.degree. C., mixed, if desired, with a polycarboxylic acid anhydride, preferably trimellitic acid anhydride,
(b) 5 to 30 wt.-%, preferably 6 to 18 wt.-%, of polyoxazolines containing at least 2 oxazoline groups per molecule, the polyoxazolines being suspended in the polyesters,
(c) 0 to 4 wt.-%, preferably 1.5 to 3 wt.-%, of paraffin, and
(d) optionally additional 1 to 50%, preferably 1 to 20%, of the total weight of the components, or reactive or nonreactive diluents and/or additives and/or fillers.
It is known to react carboxyl groups with oxazoline groups with the opening of the oxazaline ring and addition, to form an ester amide group: ##STR1## In this equation, R and R' are any desired organic moieties, such as aliphatic or aromatic moieties.
Above 120.degree. C., reaction 1 takes place at a rate that is sufficient for preparation purposes. For technical purposes, it has been utilized for lengthening the chain of polyesters (European publication OS No. 0 020 944, Japanese publications OS No. 56-18618 (1981), OS No. 55-145733 (1980)), for the preparation of block copolyesters (Japanese publication OS No. 57-36119 (1982)), for the stabilization of polyesters by reduction of the acid number, corresponding to an end-group termination (Japanese publication OS No. 55-161824 (1980)), and for the introduction of reactive flame retardants into molding compositions containing carboxyl groups (Japanese publication OS No. 57,47327 (1982)), but mainly in the thermal crosslinking of powder varnish systems for electrostatic coating (German publications OS No. 23 28 012 and OS No. 30 28 477, and Japanese publication OS No. 56-36551 (1981)).
In the latter case, a preferably branched polyester having more than 2 carboxyl groups per molecule is physically mixed with a bifunctional oxazoline by coextrusion in the molten state (followed by rapid cooling to prevent premature crosslinking), and, after grinding to a powder and electrostatic application to metal surfaces, hardened by crosslinking. Only those powdered varnish systems have been described whose polyester component decidedly changes to the plastic or liquid state above room temperature. Such systems have a long shelf life at room temperature without the occurrence of crosslinking due to reaction of the carboxyl groups with the oxazoline groups. The reason for the suppression of the crosslinking reaction, and hence for the shelf life required even of powdered varnishes, is the high glass transition temperature of the systems, which is definitely above room temperature. This can be seen by comparing them with epoxy-hardened powdered varnish systems in which, instead of the polyfunctional oxazolines, polyfunctional epoxides are made to react with the polyesters containing carboxyl groups.
The polyester components of the above-named powdered varnish systems are built up on the basis of terephthalic and isophthalic acid as well as trimellitic acid as the polycarboxylic acids, and 2,2-dimethyl-1,3-propanediol (neopentyl glycol, NPG), ethylene glycol, and 2-ethyl-2-hydroxymethyl-1,3-propanediol (trimethylol propane, TMP) as the polyols, and they have glass transition temperatures ranging from 55.degree. to 65.degree. C. This makes it possible, since molecular movement is frozen, for carboxyl and epoxide groups to coexist at room temperature without any perceptible reaction. If, however, compounds having several epoxy groups are incorporated into polyesters which are semisolid or liquid at room temperature and contain carboxyl groups, and which have a glass transition temperature below room temperature, a reaction will take place between the carboxyl groups and the epoxy groups and the system will tend to set prematurely within a few days to a few weeks, depending on the functionality of the reacting groups, and the presence, kind and quantity of a catalyst.
It was the object of the invention to develop thermosetting binder systems usable for adhesive and sealing compositions, which were to satisfy the following requirements:
1. Mixtures were sought which are poor in or are entirely free of volatile solvents.
2. The consistency of the products should be pasty or highly viscous so as to permit working at room temperature or moderately elevated temperature (up to 100.degree. C.).
3. Another requirement for the adhesive and sealing system is that it should be a single-component system, i.e., the reacting components must remain in the reactive state, in the form of a ready-to-use mixture, for months at room temperature, without any progressive crosslinking reaction that might lead to any substantial reduction of the working and service characteristics of the system.
On the basis of the great variety of the polyesters, i.e., the multiplicity of the components that can be used in preparing them, there is also a broad range of characteristics of the set products. Accordingly, a very wide range of applications was desired, such as, for example, structural adhesives for metalworking, adhesive and sealing compositions such as those used in automotive manufacture, but also low-tack products suitable for use as pressuresensitive adhesives.